The present invention relates to an electrostatic coating device and an electrostatic coating system.
The principle of electrostatic coating is to allow charged coating particles to be electrostatically adsorbed by a workpiece. Coating materials include liquid coating materials and powder coating materials. Electrostatic coating devices for liquid coating materials are classified into two types. One type is a spray gun type, and the other type is a rotary atomization type.
An electrostatic coating device of the rotary atomization type has a rotary atomization head and scatters a coating material from an outer circumferential edge of the rotating atomization head to form fine coating particles.
The electrostatic coating devices use a direct current (DC) high voltage for negatively charging coating particles. Known systems of negatively charging coating particles include an indirect charging system applying a DC high voltage to an external electrode, a direct charging system applying a DC high voltage to the rotary atomization head, etc.
To allow the coating material discharged by a coating device to be adsorbed by a workpiece without waste, it is effective to reduce a distance between the coating device and the workpiece. However, bringing the coating device close to the workpiece causes the risk of an electric discharge between the coating device and the workpiece.
An electrostatic coating system is known that has a safety circuit for preventing occurrence of an abnormal state associated with overcurrent (Japanese Laid-Open Patent Publication Nos. 2010-22933, Hei2-298374, and Hei8-187453). The safety circuit is grounded via a bleeder resistance. The safety circuit of this type monitors a current flowing between the electrostatic coating device and a workpiece and, when overcurrent is detected, the safety circuit can interrupt the high voltage applied to the electrostatic coating device and release a residual electric charge in the electrostatic coating device via the bleeder resistance to a ground at the same time, thereby reducing the electrical potential of the electrostatic coating device to a safe level.
However, the releasing of the residual electric charge through the bleeder resistance is limited in discharge speed. In particular, when coating is performed at a short distance between the electrostatic coating device and the workpiece and the safety circuit detects an increase in high-voltage current, the electrostatic coating device tends to instantaneously discharge the accumulated charge toward the workpiece before the supply of the high voltage is interrupted and the residual electric charge is discharged to the ground at the same time by the operation of the safety circuit. A proposal for improvement in this problem is made in Japanese Laid-Open Patent Publication No. Hei8-187453. Japanese Laid-Open Patent Publication No. Hei8-187453 proposes a ring electrode disposed at a leading end of a shaping air ring so as to charge coating particles with this ring electrode.
Japanese Laid-Open Patent Publication No. 2000-117155 proposes a rotary atomization type electrostatic coating device preventing spark discharge between a workpiece and the electrostatic coating device. FIG. 9 accompanying the description of this application corresponds to FIG. 2 of Japanese Laid-Open Patent Publication No. 2000-117155. Referring to FIG. 9 accompanying the description of this application, reference numeral 200 denotes a rotary atomization type electrostatic coating device and FIG. 9 shows a front end portion of the electrostatic coating device 200. Reference numeral 202 denotes a rotary atomization head. The rotary atomization head 202 is fixed to a front end portion of a hollow rotary shaft 204. The hollow rotary shaft 204 is driven by an air motor 206. In FIG. 9, only a leading-end sleeve portion of the air motor 206 is shown.
A motor support case 208 surrounding the air motor 206 and a shaping air ring 210 attached to a leading end of the motor support case 208 are made of an insulating resin material. The air motor 206 is made of a conductive metal material. The hollow rotary shaft 204 is made of an insulating material, specifically, an insulating ceramic material. The rotary atomization head 202 is made of an insulating resin material.
The shown electrostatic coating device 200 employs a center feed system as a system for supplying a coating material to the rotary atomization head 202. In particular, a feed tube 212 is inserted in the hollow rotary shaft 204 and the coating material is supplied through the feed tube 212 to a center portion of the rotary atomization head 202. The feed tube 212 is made of an insulating resin material.
The electrostatic coating device 200 has a high-voltage generator built-in. This built-in high-voltage generator is referred to as “a cascade”. The high voltage of −60 kV to −120 kV generated by the cascade is supplied to the air motor 206. A path supplying the high voltage from the air motor 206 to the rotary atomization head 202 is configured as follows.
A first semiconductive film 204a is formed on an outer circumferential surface of the hollow rotary shaft 204. A second semiconductive film 202a is formed on an outer circumferential surface of the rotary atomization head 202. The second semiconductive film 202a extends to an outer circumferential edge 202b of the rotary atomization head 202.
A gap 214 is formed between a leading end of the air motor 206 and a rear end of the rotary atomization head 202. First and second circular-arc films 216a, 218a formed on outer circumferential surfaces of first and second limiting rings 216, 218 are disposed at both axial ends of the gap 214. The first and second circular-arc films 216a, 218a are made of a semiconductive material.
A high voltage application path from the air motor 206 to the rotary atomization head 202 is made up of the first circular-arc film 216a, the first semiconductive film 204a of the hollow rotary shaft 204, the second circular-arc film 218a, and the second semiconductive film 202a of the rotary atomization head 202. The high voltage passing through this high voltage application path is supplied to an end of the second semiconductive film 202a of the rotary atomization head 202, i.e., the outer circumferential edge 202b of the rotary atomization head 202. This outer circumferential edge 202b acts as a discharge electrode.
According to the rotary atomization type electrostatic coating device 200 of Japanese Laid-Open Patent Publication No. 2000-117155, when the rotary atomization head 202 comes abnormally close to a workpiece, the residual electric charge in the air motor 206 made of conductive metal is dispersed by resistances of the portions 216a, 204a, 218a, 202a made up of semiconductive films. As a result, a discharge energy can be kept smaller. Additionally, even when the rotary atomization head 202 short-circuits with a workpiece, spark discharge can be prevented from occurring.
Moreover, even when the rotary atomization head 202 comes rapidly and abnormally close to a workpiece, the first limiting ring 216 disposed at the leading end side of the air motor 206 can alleviate concentration of an electric field at the leading end of the air motor 206. Similarly, the second limiting ring 218 disposed at the rear end side of the rotary atomization head 202 can alleviate concentration of an electric field at the rear end of the rotary atomization head 202.